U.S. Pat. No. 4,084,024 which issued Apr. 11, 1978 in the name of Joseph C. Schumacher, and which is assigned to the present assignee, discloses and claims a process for the production of semiconductor grade silicon using hydrogen reduction at relatively high temperatures, for example, within a temperature range of from 900.degree. C.-1200.degree. C. The process of the present invention, on the other hand, involves a process for producing a semiconductor grade silicon involving the use of thermal decomposition which is carried out at a lower and more economical temperature range of, for example, 500.degree. C.-900.degree. C.
As pointed out in the patent, recent developments in the semiconductor industry have created a growing demand for a low cost single crystal silicon of extremely high purity, which is known as semiconductor grade silicon, and which is used in the manufacture of semiconductor devices and silicon photovoltaic solar cells. For that reason, a multitude of prior art processes have been conceived for the production of semiconductor grade silicon, including the process covered by the patent. The prior art processes can be classified into the following six basic approaches:
1. The Siemens process described in GDR Pat. Nos. 1,066,564; 1,102,117; 1,233,815 and British Pat. No. 904,239 by which essentially all current semiconductor grade polycrystalline silicon is produced, is expressed by the following chemical reaction. ##STR2## PA1 2. Silicon tetrachloride-hydrogen reduction is utilized in some cases because of the availability of byproduct SiCl.sub.4 from the Siemens process. An alternative SiCl.sub.4 production reaction is included here since it may be used as a source. ##STR3##
This is a high temperature batch process providing heterogeneously nucleated silicon growth on heated Si filaments and large volumes of SiCl.sub.4 and explosive polymeric byproducts which must be disposed of. The process is as a result of these byproducts, not a closed-loop process. In addition, a 20/1 excess of H.sub.2 over stoichiometry is required.
This again is a high temperature, non-closed-loop, batch process providing heterogeneously nucleated growth on a heated substrate and requires a large H.sub.2 excess.
3. The DuPont process as described in U.S. Pat. Nos. 3,012,862 and 4,084,024 where in SiX.sub.4 or SiHX.sub.3 (where X=Cl,Br,I) is reduced in a fluid or moving bed by H.sub.2, Zn, or Cd. The reaction chemistry is as follows:
I. Feed preparation EQU (MG)Si+3HX.fwdarw.SiHX.sub.3 +SiX.sub.4 +H.sub.2 etc.
with SiX.sub.4 here a byproduct
or EQU (MG)Si+2X.sub.2 .fwdarw.SiX.sub.4
II. Ultrapure silicon production by ##STR4##
These are moderately high temperature, non-closed loop processes with the byproducts varying the particular process chemistry, and which require large hydrogen excesses where it is used. However, it is reported in U.S. Pat. No. 3,012,862 that large quantities of amorphous silicon is produced in the halosilane decomposition which is a finely dispersed powder and which must be avoided if the apparatus is to operate properly without becoming plugged and which must be removed from the end product. Attempts are made to avoid the formation of amorphous silicon by introducing a diluent into the process to dilute the silane; which at best merely suppresses to some extent the formation of amorphous silicon and which requires extraneous equipment. It is also suggested that the formation of the amorphous silicon can be suppressed by operating the reactor at a vacuum pressure which likewise, requires extraneous equipment and which creates sealing problems. The process of the present invention is unique in that it uses undiluted tribromosilane at standard atmospheric pressure (14.7 p.s.i.A.) or above, up to, for example, 50 p.s.i.A, and yet eliminates completely the formation of amorphous silicon product.
4. The Iodide process described in U.S. Pat. No. 3,020,129 expressed as follows: EQU Si+2I.sub.2 .fwdarw.SiI.sub.4
and thermal decomposition to produce Si ##EQU1##
This is a moderate-temperature closed-loop batch process in which polycrystalline or single crystal silicon is grown on a seed particle or heated filament.
5. The Union Carbide process expressed as follows: Tricholorosilane preparation ##STR5## Ion exchange redistribution to silane according to ##STR6## with appropriate byproduct recycle followed by silane thermal decomposition. ##STR7##
This is a low temperature, closed-loop process involving an ion exchange intermediate redistribution and produces homogeneously nucleated product.
6. The thermal decomposition of trichlorosilane according to ##STR8## is described in U.S. Pat. Nos. 2,943,918 and 3,012,861. Presumably the trichlorosilane is prepared according to EQU Si+HCl.fwdarw.SiHCl.sub.3 +SiCl.sub.4 +other products
so that a non-closed process would result. Only batch type operation is proposed to promote heterogenous nucleation and homogeneous nucleation is avoided and thought harmful.
Many other techniques and slight modifications of the techniques presented are contained within the prior art, however, none would appear to have a material bearing on the present invention.
An important feature of the process of the invention is that it is a continuous process unlike the prior art batch process 1, 2, 4 and 6 described briefly above. As is well known, the continuous process represents an improvement over the batch processes in the reduction of capital costs and operating expenses per unit of product.
Another important feature of the process of the present invention is that it is a closed-loop low temperature process; whereas the prior art processes 1, 2, 3 and 6, supra, are high temperature, open-loop processes. The prior art processes represent higher operating expenses due to their excessive energy requirements and the need for the disposal of corrosive and hazardous byproducts.
Another feature of the process of the invention is that it utilizes a direct high yield thermal decomposition of tribromosilane in contrast to the low yield thermal decomposition process of U.S. Pat. Nos. 2,943,918 and 3,012,861, rather than going through the ion exchange redistribution of prior art process 5 in order to obtain a material suitable for thermal decomposition. The inherent simplicity of the process of the present invention results in a reduction in complexity and operating costs and an improvement in yield capabilities.
Another important feature of the process of the invention is the avoidance of wall build-up in the thermal decomposition reaction by maintaining a critical temperature differential between the bed and the surrounding walls.